It is a problem in the field of medical monitoring instruments to manufacture a photoplethysmographic probe that satisfies a number of diverse and sometimes contradictory requirements. It is important that the probe both be simple to use and conform to a variety of patients who differ in size and shape. The probe must be securely affixable to the patient, such as on a patient's appendage, without requiring complex structures or elements that can irritate the patient. In addition, in order to reduce the risk of infection and contamination, at least a portion of the probe should be built to be disposable so that the probe is used one or more times with the patient and can then be destroyed. The disposable portion of the probe must be inexpensive so that it can be disposable after use and yet the patient must be shielded from any potentially dangerous electrical signals or heat produced by the probe. The probe must also reliably and accurately perform the required blood analyte measurements. The probe, cable and monitoring instrument are all subjected to a hostile environment and must be manufactured to be rugged to survive rough handling and the presence of highly reactive fluids.
Another problem with present photoplethysmographic probes is the proliferation of probe types and monitor models. The number of manufacturer designs probes for use with their specific monitors. Also, the types of photoplethysmographic monitors continues to increase. One of the primary uses of photoplethysmography has been the monitoring of the oxygen saturation of a patient's blood. However, there is a desire to expand the use of photoplethysmography into the monitoring of additional blood analytes such as carboxyhemoglobin, methemoglobin and other dyshemoglobins. This proliferation of monitor types will add additional confusion in the health care environment due to the number of additional probe types which will be offered with these new monitors.
In the specific field of pulse oximetry, the light beams are typically generated by a probe using light emitting diodes (LEDs) that produce light beams at red and infrared wavelengths. Various manufacturers use different wavelengths of LED's in illuminating the tissue of a patient. Additionally, many manufacturers use LED's which produce light having a spectral content characterized by a center wavelength which varies from the nominal wavelength of the LED. Therefore, many photoplethysmographic probes use one or more means for identifying characteristics about the probe being used. One common identification means is a resistor which resides in the probe and identifies the spectral characteristics of the emitters being used in the probe thereby enabling the photoplethysmographic monitor to utilize the correct calibration data when generating the blood analyte level. Another use of the identification means is identifying the type or manufacturer of a probe. Presently, however, there are no probes which can universally be used with all manufacturer's phtotoplethysmograhpic monitors.
In future photoplethysmographic systems it may be preferable to use laser diodes, which produce a beam of substantially monochromatic light at or exceeding the light power available from light emitting diodes that are typically used in photoplethysmography. The difficulty with laser diodes is that their cost currently prevents them from being used in a disposable probe. Placement of the laser diode in the monitoring instrument necessitates the use of one or more fiber optic strands in the cable that interconnects the disposable probe with the monitoring instrument. The cable in a hospital environment typically suffers rough handling and the life of the fiber optic strands in the connector cable can be fairly limited, thereby increasing the effective cost of the disposable probe since the cable must typically be replaced on a fairly frequent basis.
Alternatively, the laser diodes can be placed in a segment of the interconnect cable between the monitor and the patient end of the probe. This, however, still requires that the segment of the interconnect cable housing the laser diodes be reusable in order to reduce costs.